Conventional gas turbine engines are widely known to include means for bleeding off a portion of the airflow through the compressor to provide cooling air which is ducted through internal components and maintains such components cool relative to the hot annular gaspath. The cooling air is ducted through or over components to cool them and is directed further to be dispersed into the hot gaspath flow before being rearwardly expelled from the turbine engine.
The separation of internal cooling airflow and external gaspath flow is accomplished with running seals between stationary engine components and rotating assemblies. The running seals allow passage of a controlled leakage flow of cooling air in a leakage pathway between the stator and rotor assemblies to purge any hot gaspath air which would otherwise migrate into this area, and heat the adjacent internal components.
Conventionally the blades of the stators and rotors have platforms which step or overlap rearwardly to direct hot gaspath airflow axially rearwardly. This configuration has been relied upon conventionally to impede migration of hot airflow into the cooling air leakage pathway between the rotor and stator assemblies, and to direct cooling air leakage flow generally into the gaspath at an acute angle relative to rearwardly axial flow therein.
An example of such a conventional turbine coolant flow system is described in U.S. Pat. No. 3,609,057 to Radtke issued Sep. 28, 1971 and which can be considered well known to those skilled in the art.
In U.S. Pat. No. 5,211,533 to Walker et al, the reintroduction of cooling airflow passing by a running seal and into the compressor flow (upstream of the burners and gaspath) is controlled with a flow diverter. This diverter comprises a curved plate mounted forward of a stator blade and redirects cool airflow over the stator blade platform into the annular airflow passage within the turbine compressor section at an acute angle to the annular airflow direction.
When reintroducing the leaked cooling air towards a static blade there is no centrifugal force component to address and disturbance of the airflow within the annular passage is relatively low. Since rotor blades rotate at high angular speeds, for example 30,000 rpm, the effect of radial forces is substantial when considering reintroduction of cooling air.
In such an environment, the prior art either neglects to recognize the importance or does nor consider the effect of centrifugal forces on the reintroduced cooling air as in U.S. Pat. No. 3,609,057 to Radtke. Other approaches to this problem rely on improving seals or bypass the rotor area.
For example, U.S. Pat. No. 4,507,052 to Thompson uses six circumferential ridges on a running seal to better prevent leakage between the stator and rotor assemblies. A forward seal is used to prevent cooling air flow from the rear plenum and manifolds from merely passing through to the forward face of the rotor. However, no means is provided to prevent leakage air from passing through the six ridge running seal into gaps between the blade platforms and airfoils of adjacent blades. Such leakage air would enter the gaps between blades and be propelled radially into the gaspath by the centrifugal force of the rapidly rotating rotor blades.
Other examples of leakage air treatment involve use of a labyrinth to impede leakage air flow as in U.S. Pat. No. 5,252,026 to Shepherd, or use of labyrinth combined with bypass conduits as in U.S. Pat. No. 4,348,157 to Campbell et al.
However, none of the prior art methods of reintroducing cool leakage air back into the hot gaspath address the problem of cool air entering the gaps between rapidly rotating rotor blades and being expelled under centrifugal force into the gaspath in a transverse direction. It has been found by the inventors that such reintroduction significantly disturbs the gaspath flow and reduces engine efficiency.